1. Field of the Invention
The invention relates to the field of fasteners and, in particular, to a non-pyrotechnic fastener that automatically separates a nut from a bolt upon actuation.
2. Description of Related Art
Reliable fasteners that separate upon actuation have many applications. One critical application is on launch vehicles designed to place a spacecraft into orbit. Not only must the fasteners reliably secure booster stages together under high loads, they must rapidly separate upon actuation in order to achieve proper timing of stage separation. This is particularly true when several fasteners must be simultaneously separated. Thus pyrotechnically actuated devices are typically used. An extreme example is an explosive actuated system that uses a metal coupling to join the segments of the fairing together. A tubular member is positioned next to or within the coupling. Upon ignition, the explosive expands the tubular member, which in turn fractures the coupling. Such a system is disclosed in U.S. Pat. No. 5,443,492 xe2x80x9cPayload Housing And Assembly Joint For A Launch Vehiclexe2x80x9d by A. L. Chan, et al.
However, pyrotechnic fasteners and the like, while well proven, can not be tested prior to use, thus must be assembled with great care. This makes them generally expensive to manufacture. Special storage areas must be set aside for any device containing explosives. They are always subject to inadvertent actuation, and, therefore, handled with great care. Additionally, they are particularly subject to ignition by electromagnetic interference (EMI) and thus must be protected by EMI shielding devices, which also raises the cost. One of the most important disadvantages is that upon actuation, most generate significant shock loads, which can damage nearby equipment.
One approach to eliminate such problems is to use shape memory alloys to actuate the fasteners. Shape memory alloys offer a solution to the problem. There are basically two types of shape memory alloys:
1. Simple memory alloys where a deformation undergone in an austenitic state is definitively cancelled out during the passage to the austenitic state.
2. Reversible memory alloys where a deformation undergone in the martensitic state is cancelled out during the passage into the austenitic state, but is reassumed during a subsequent passage to the martensitic state. However, the transformation takes place with a certain hysteresis.
There are numerous alloys having shape memory characteristics such as Tixe2x80x94Ni, Auxe2x80x94Cd, Inxe2x80x94Zn, Tixe2x80x94Nixe2x80x94Cu, Cuxe2x80x94Znxe2x80x94Al and Cuxe2x80x94alxe2x80x94Ni, and many are commercially available. The theory of shape memory alloys is well established and, therefore, need not be discussed in further detail.
There are many examples of fasteners making use of a shape memory alloy (SMA). For example, U.S. Pat. No 5,312,152 xe2x80x9cShape Memory Metal Actuated Separation Devicexe2x80x9d by W. H. Woebkenberg, Jr., et al. uses a segmented nut that is kept in engagement with a threaded bolt by a retainer. The retainer is held in place by a SMA element. Upon heating of the SMA element, it returns to its un-deformed state and releases the retainer, which in turn releases the nut. U.S. Pat. No 5,722,709 xe2x80x9cSeparation Device Using A Shape Memory Alloy Retainerxe2x80x9d by B. K. Lortz also uses a segmented nut. However, in this case the nut is retained in contact with the threaded bolt by a SMA collar. Upon heating, it expands to its original shape releasing the segmented nut. Other examples of fasteners using shape memory alloys can be found in U.S. patent applications Ser. No. 5,060,888 Temporary Linking Device Especially For An Artificial Satellite Lengthening Piece, And Method To Free Such A Linkxe2x80x9d by G. Vezain, et al., U.S. Pat. No. 5,129,753 xe2x80x9cShape Memory Wire Latch Mechanismxe2x80x9d by K. S. Wesley, et al., U.S. Pat. No. 5,150,770 xe2x80x9cRecharge Device, Particularly For Drive Mechanisms For Extending And Withdrawing Operative Members Of A Space Vehiclexe2x80x9d by G. Secci and U.S. Pat. No. 5,718,531 xe2x80x9cLow Shock Release Devicexe2x80x9d by E. C. Mutschleer, Jr. All use SMA materials as the primary actuating force. However, when using SMA material as the primary actuating device, precise timing of the release can prove difficult to achieve. In addition, shape memory alloys are sensitive to high temperature environments.
Another approach is the use of ball latches. U.S. Pat. No. 3,887,150 xe2x80x9cInternal Ejector Mechanismxe2x80x9d by T. Jakubowski, Jr., 132,147 xe2x80x9cStore Retention And Release Mechanismxe2x80x9d by A. Contaldo, U.S. Pat. No. 4,350,074 xe2x80x9cMechanical And Electrical Coupling Device Fore Charges, Particularly Military Chargesxe2x80x9d by J. P. Rouget, et al., U.S. Pat. No. 4,520,711 Loop Retention Device For Hook Operated Bomb Arming Solenoidsxe2x80x9dby P. R. Robinson, U.S. Pat. No. 5,364,046 xe2x80x9cAutomatic Compliant Capture And Docking Mechanism for Spacecraftxe2x80x9d by M. E. Dobbs, et al., and U.S. Pat. No. 5,520,476 xe2x80x9cTie-Down And Release Mechanism For Spacecraftxe2x80x9d by G. W. Marks, et al. all disclose the use of ball dxc3xa9tente mechanisms to secure components of one type or another together. The main problem with such ball latch fasteners is limited trigger force reduction, which is required for activation with SMA systems. In launch vehicles and spacecrafts, which are subjected to very large vibration loads, the satellite(s) must be secured using very high pre-loaded joints. Ball latch systems typically don""t allow for the application of the type of pre-loads that can be obtained with a threaded fastener. However, they are very good locking devices.
In U.S. Pat. No. 5,603,595 xe2x80x9cFlywheel Nut Separable Connector And Methodxe2x80x9d by W. D. Nygren, an attempt was made to take advantage of SMA technology to provide actuation initiation for a conventional nut and bolt, and to use the high pre-load forces therebetween to provide the primary separation forces, i.e. to rotate the nut to the point of separation. The nut, having a high helix angle or lead, is essentially a flywheel. It is torqued until the desired preload is achieved. Thereafter, the flywheel is latched. The latch is secured by a SMA spring. Upon heating the spring, the latch releases the flywheel, and the stored energy therein tends to cause the flywheel to initially rotate at high speed. The strain energy due to the pre-load is dissipated as the nut unwinds, and the stored energy in the flywheel continues to cause the nut to rotate until separation occurs. The advantages are numerous; high pre-loaded joints are possible, and the need to only heat a small wire spring greatly reduces actuation time. However, this design had problems in that it had a greater parts count than equivalent explosive actuated separation nuts and was somewhat more massive and occupied more volume.
Thus it is a primary object of the invention to provide a fastener assembly that automatically separates upon actuation.
It is another primary object of the invention to provide a non-pyrotechnically actuated fastener assembly.
It is a further object of the invention to provide a fastener assembly that automatically separates upon actuation and absorbs the stored energy produced by the pre-loading of the fastener to reduce shock loads.
It is a still further object of the invention to provide a fastener assembly that automatically separates upon actuation and is easily re-settable.
It is a still further object of the invention to provide a fastener assembly that has low-mass, volume and parts count.
The invention is a separable connector assembly for joining two surfaces together. In detail, the invention includes a first fastener half, typically a nut, translationally mounted to a first surface. The first fastener half includes a threaded end with a selected thread geometry including a selected thread pitch diameter, thread lead angle, and helix angle. Preferably, the selected helix angle is between 18 degrees and 45 degrees, the selected thread angle is between 0 degrees and 30 degrees (7 degrees is preferred), and the selected thread lead is between 0.5 thread pitch diameters and 1.5 thread pitch diameters.
A hollow first housing having a cylindrical wall with a specific thickness is mounted to a second surface. A second fastener half, typically a bolt, is rotatably mounted within the hollow first housing. This second fastener half is generally threadably engagable with the threaded end of the first fastener half. A first mechanism or means is included for releasably restraining the rotatably supported second fastener half from rotating. It includes a cylindrical wall having a plurality of rectangular slots. A plurality of cylindrical rollers is generally movably mounted in the slots. These rollers generally have a diameter greater than the thickness of the cylindrical wall of the first housing. The second fastener half includes a plurality of cylindrical grooves having a depth less than the diameter of the cylindrical rollers. These grooves are generally alignable with the slots in the second fastener half. A coil spring (wrap spring) is wound about the cylindrical wall of the first housing. This coil spring is at least generally movable from a first position to a second position. In the first position, the coil spring generally engages the rollers forcing the rollers into the grooves locking the body to the cylindrical wall of the first housing. In the second position, the coil spring generally allows the rollers to move out of the slots in the second fastener half.
A second mechanism or means is mounted on the first housing to wind the spring about the cylindrical wall of the first housing such that the spring is moved from the second position to the first position. Preferably, the second mechanism includes a circular shaped ratchet assembly mounted about the first housing. The ratchet assembly includes a first member having flexible pawl springs rigidly attached to the first housing and a second member in the form of a ratchet rotatably mounted to the first housing and attached to the first end of the spring. A third mechanism releasably restrains the second end of the wrap spring from moving. Thus, when the third mechanism restrains the second end of the spring and when the ratchet member of the ratchet assembly is rotated, the wrap spring is wound about the first housing from the second position to the first position thereof.
The third mechanism or means is mounted on the first housing and coupled to the second end of the spring at least when the spring is in the first position. When the third mechanism releases the spring, the spring can move to the second position. Preferably, the third mechanism includes a shaft rotatably mounted within the first housing movable from a first position to a second position, the shaft having a cam surface that restrains the second end of the spring when the shaft is in the first position and releases the second end of the spring when the shaft is in the second position. A shaft lever is attached to the shaft for moving the shaft from the first position to the second position. When in the first position, the wrap spring biases the shaft lever to rotate the shaft lever to the second position. A balanced latch lever and spring assembly secures the lever such that the shaft is in the first position. The balanced latch lever is acted upon by SMA wires. Upon heating by the application of electrical current, each of the SMA wires returns to its original shortened length rotating the balanced latch lever and so releases the shaft lever releasing the shaft and, of course, the wrap spring.
A fourth mechanism or means is provided for restraining the threaded end of the first fastener half from rotating. Preferably, this fourth mechanism or means comprises a hollow second housing mountable to the first surface, wherein this hollow second housing comprises an opening having a cross-section configured for receiving the non-circular cross-sectional portion of the first fastener half. Finally, a fifth mechanism or means is mounted on the first surface for applying a selected tensile load to the first fastener half. Preferably, this fifth mechanism or means comprises the first fastener half having a flange, and a plurality of set screws threadably mounted in the flange and engagable with the second housing. Thus, when the first fastener half is threadably engaged with the second fastener half, the set screws can be adjusted to engage the second housing causing the first and second fastener halves to be strained.
In addition, a sixth mechanism or means may be included for releasably securing the shaft of the third mechanism or means in the first location. Preferably, this sixth mechanism or means comprises a lever arm attached to the shaft, a balanced latch lever releasably engaging the lever arm, a biasing means (e.g., a compression spring) to urge the balanced latch lever into engagement with the lever arm, and a seventh mechanism or means for moving the balanced latch lever out of engagement with the lever arm. This seventh mechanism or means preferably comprises a shape memory wire attached between the balanced latch lever and the first housing such that when the wire is heated, the wire shortens causing the balanced latch lever to rotate out of contact with the lever arm.
Thus, when the first fastener half and the second fastener half are engaged to form a connection, and with a tensile load applied to the joined first fastener half causing the connection to be strained, the selected thread geometry causes the tensile load to be resolved as a torque applied to the second fastener half. This torque is generally sufficient to cause the second fastener half to rotate when released allowing the threaded end to translate out of engagement with the rotatably supported second fastener half when the mechanism releases the second fastener half allowing the connection to separate.
Preferably, the rotatably supported second fastener half has a selected mass moment of inertia and the selected thread geometry is such that less than 10 percent of the strain energy stored in the connection between the first fastener half and the rotatably supported second fastener half, not dissipated as heat due to friction, is converted into translational kinetic energy of the first fastener half during separation.
The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description in connection with the accompanying drawings in which the presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for purposes of illustration and description only and are not intended as a definition of the limits of the invention.